Distributed Power Harvesting Systems Using DC Power Sources

ABSTRACT

A photovoltaic panel with multiple photovoltaic sub-strings including serially-connected photovoltaic cells and having direct current (DC) outputs adapted for interconnection in parallel into a parallel-connected DC power source. A direct current (DC) power converter including input terminals and output terminals is adapted for coupling to the parallel-connected DC power source and for converting an input power received at the input terminals to an output power at the output terminals. The direct current (DC) power converter optionally has a control loop configured to set the input power received at the input terminals according to a previously determined criterion. The control loop may be adapted to receive a feedback signal from the input terminals for maximizing the input power. A bypass diode is typically connected in shunt across the input terminals of the converter. The bypass diode functions by passing current during a failure of any of the sub-strings and/or a partial shading of the sub-strings. The bypass diode may be a single bypass diode connected across the parallel-connected DC power source. The DC power converter may convert the input power at high current to the output power at a lower current. The output terminals may be connectible with wire cables to a load, and the DC power converter is configured to reduce energy loss through the wire cables to the load.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part application of U.S.patent application Ser. No. 11/950,271 filed Dec. 4, 2007 by the sameinventors.

BACKGROUND 1. Field of the Invention

The field of the invention relates generally to power production fromdistributed DC power sources, and particularly to a photovoltaic panelhaving series connected photovoltaic strings the outputs of which areconnected in parallel.

2. Related Arts

The recent increased interest in renewable energy has led to increasedresearch in systems for distributed generation of energy, such asphotovoltaic cells (PV), fuel cells, batteries (e.g., for hybrid cars),etc. Various topologies have been proposed for connecting these powersources to the load, taking into consideration various parameters, suchas voltage/current requirements, operating conditions, reliability,safety, costs, etc. For example, most of these sources provide lowvoltage output (normally a few volts for one cell, or a few tens ofvolts for serially connected cells), so that many of them need to beconnected serially to achieve the required operating voltage.Conversely, a serial connection may fail to provide the requiredcurrent, so that several strings of serial connections may need to beconnected in parallel to provide the required current.

It may be also known that power generation from each of these sourcesdepends on manufacturing, operating, and environmental conditions. Forexample, various inconsistencies in manufacturing may cause twoidentical sources to provide different output characteristics.Similarly, two identical sources may react differently to operatingand/or environmental conditions, such as load, temperature, etc. Inpractical installations, different source may also experience differentenvironmental conditions, e.g., in solar power installations some panelsmay be exposed to full sun, while others be shaded, thereby deliveringdifferent power output. In a multiple-battery installation, some of thebatteries may age differently, thereby delivering different poweroutput. While these problems and the solutions provided by the subjectinvention may be applicable to any distributed power system, thefollowing discussion turns to solar energy so as to provide betterunderstanding by way of a concrete example.

In view of the above, a newly proposed topology for connecting multipleDC power sources to the load should also lend itself to easy testing andoperational verification during and after installation.

BRIEF SUMMARY

According to exemplary aspects there may be provided a photovoltaicpanel with multiple photovoltaic sub-strings includingserially-connected photovoltaic cells and having direct current (DC)outputs adapted for interconnection in parallel into aparallel-connected DC power source. A direct current (DC) powerconverter including input terminals and output terminals may be adaptedfor coupling to the parallel-connected DC power source and forconverting an input power received at the input terminals to an outputpower at the output terminals. The direct current (DC) power convertermay have a control loop configured to set the input power received atthe input terminals according to a previously determined criterion. Thecontrol loop may be adapted to receive a feedback signal from the inputterminals for maximizing the input power. A bypass diode may beconnected in shunt across the input terminals of the converter. Thebypass diode functions by passing current during a failure of any of thesub-strings and/or a partial shading of the sub-strings. The bypassdiode may be a single bypass diode connected across theparallel-connected DC power source. The DC power converter may convertthe input power at high current to the output power at a lower current.The output terminals may be connectible with wire cables to a load, andthe DC power converter may be configured to reduce energy loss throughthe wire cables to the load.

According to further aspects there may be provided a distributed powerharvesting system including a photovoltaic panel having multiplesub-strings including serially-connected photovoltaic cells. Thesub-strings may be connectible in parallel to provide aparallel-connected DC power source. A power converter has inputterminals which may be adapted for attaching to the parallel-connectedDC power source. A bypass diode may be typically connected across theparallel-connected DC power source. A load may be connectible to theoutput terminals. The power converter may include a circuit loopconfigured to set input power received at the input terminals accordingto a previously determined criterion. The previously determinedcriterion may be usually a maximum input power.

According to further aspects, a method for power harvesting in a systemincluding a photovoltaic panel with multiple sub-strings and a powerconverter. The sub-strings may be serially-connected photovoltaic cells.The power converter has input terminals and output terminals. Thesub-strings may be connected in parallel to provide a parallel-connectedDC power source. The input terminals of the converter may be attached tothe parallel-connected DC power source. A single bypass diode may betypically attached across the parallel-connected DC power source. Powermay be converted by the power converter from the input terminals to theoutput terminals. During the conversion, the input power received at theinput terminals may be set according to a previously determinedcriterion. The output terminals may be connected to a load. Whileconverting power, the current output from the output terminals may bereduced, thereby reducing energy losses.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, exemplify the embodiments of the presentinvention and, together with the description, serve to explain andillustrate principles of the invention. The drawings are intended toillustrate major features of the exemplary embodiments in a diagrammaticmanner. The drawings are not intended to depict every feature of actualembodiments nor relative dimensions of the depicted elements, and arenot drawn to scale.

FIG. 1 illustrates a conventional centralized power harvesting systemusing DC power sources.

FIG. 2 illustrates current versus voltage characteristic curves for oneserial string of DC sources.

FIG. 3 illustrates a distributed power harvesting system, according toaspects of the invention, using DC power sources.

FIGS. 4A and 4B illustrate the operation of the system of FIG. 3 underdifferent conditions, according to aspects of the invention.

FIG. 4C illustrates an feature of the invention wherein the invertercontrols the input current.

FIG. 5 illustrates a distributed power harvesting system, according toother aspects of the invention, using DC power sources.

FIG. 6 illustrates an exemplary DC-to-DC converter according to aspectsof the invention.

FIG. 7 illustrates a power converter, according to aspects of theinvention including control features of the aspects of the invention.

FIG. 8 illustrates characteristic current-voltage curves of a singlephotovoltaic cell at different illumination levels.

FIG. 9 illustrates an arrangement of a solar panel according toconventional art.

FIG. 10 shows an arrangement of a solar panel according to an feature ofthe present invention.

FIG. 11 illustrates a method illustrating various exemplary aspects ofthe present invention.

The foregoing and/or other aspects will become apparent from thefollowing detailed description when considered in conjunction with theaccompanying drawing figures

DETAILED DESCRIPTION

A conventional installation of solar power system 10 is illustrated inFIG. 1. Since the voltage provided by each individual solar panel 101may be low, several panels may be connected in series to form a stringof panels 103. For a large installation, when higher current may berequired, several strings 103 may be connected in parallel to form theoverall system 10. The solar panels may be mounted outdoors, and theirleads may be connected to a maximum power point tracking (MPPT) module107 and then to an inverter 104. The MPPT 107 may be typicallyimplemented as part of the inverter 104. The harvested power from the DCsources may be delivered to the inverter 104, which converts thefluctuating direct-current (DC) into alternating-current (AC) having adesired voltage and frequency, which may be usually 110V or 220V at 60Hz, or 220V at 50 Hz (It may be interesting to note the even in the USmany inverters produce 220V, which may be then split into two 110V feedsin the electric box). The AC current from the inverter 104 may then beused for operating electric appliances or fed to the power grid.Alternatively, if the installation is not tied to the grid, the powerextracted from the inverter may be directed to a conversion andcharge/discharge circuit to store the excess power created as charge inbatteries. In case of a battery-tied application, the inversion stagemight be skipped altogether, and the DC output of the MPPT stage 107 maybe fed into the charge/discharge circuit.

As noted above, each solar panel 101 supplies relatively very lowvoltage and current. A possible challenge facing the solar arraydesigner may be to produce a standard AC current at 120V or 220Vroot-mean-square (RMS) from a combination of the low voltages of thesolar panels. The delivery of high power from a low voltage requiresvery high currents, which may cause large conduction losses on the orderof the second power of the current (I²). Furthermore, a power inverter,such as the inverter 104, which may be used to convert DC current to ACcurrent, may be most efficient when its input voltage may be slightlyhigher than its output RMS voltage multiplied by the square root of 2.Hence, in many applications, the power sources, such as the solar panels101, may be combined in order to reach the correct voltage or current.The most common method connects the power sources in series in order toreach the desirable voltage and in parallel in order to reach thedesirable current, as shown in FIG. 1. A large number of the panels 101may be connected into a string 103 and the strings 103 may be connectedin parallel to the power inverter 104. The panels 101 may be connectedin series in order to reach the minimal voltage required for theinverter. Multiple strings 103 may be connected in parallel into anarray to supply higher current, so as to enable higher power output.

While this configuration may be advantageous in terms of cost andarchitecture simplicity, several drawbacks may be identified. Onerecognized drawback may be inefficiencies cause by non-optimal powerdraw from each individual panel, as explained below. As explained above,the output of the DC power sources may be influenced by many conditions.Therefore, to maximize the power draw from each source, one needs todraw the combination of voltage and current that provides the peak powerfor the currently prevailing conditions. As conditions change, thecombination of voltage and current draw may need to be changed as well.

FIG. 2 illustrates one serial string of DC sources, e.g., solar panels201 a-201 d, connected to MPPT circuit 207 and inverter 204. The currentversus voltage (IV) characteristics plotted (210 a-210 d) to the left ofeach DC source 201. For each DC source 201, the current decreases as theoutput voltage increases. At some voltage value the current goes tozero, and in some applications may assume a negative value, meaning thatthe source becomes a sink. Bypass diodes may be used to prevent thesource from becoming a sink. The power output of each source 201, whichmay be equal to the product of current and voltage (P=I*V), variesdepending on the voltage drawn from the source. At a certain current andvoltage, close to the falling off point of the current, the powerreaches its maximum. It may be desirable to operate a power generatingcell at this maximum power point. The purpose of the MPPT may be to findthis point and operate the system at this point so as to draw themaximum power from the sources.

In a typical, conventional solar panel array, different algorithms andtechniques may be used to optimize the integrated power output of thesystem 10 using the MPPT module 107. The MPPT module 107 receives thecurrent extracted from all of the solar panels together and tracks themaximum power point for this current to provide the maximum averagepower such that if more current may be extracted, the average voltagefrom the panels starts to drop, thus lowering the harvested power. TheMPPT module 107 maintains a current that yields the maximum averagepower from the overall system 10.

However, since the sources 201 a-201 d may be connected in series to asingle MPPT 207; the MPPT must select a single point, which would besomewhat of an average of the MPP of the serially connected sources. Inpractice, it may be very likely that the MPPT would operate at acurrent-voltage I-V point that may be optimum to only a few or none ofthe sources. In the example of FIG. 2, the selected point may be themaximum power point for source 201 b, but may be off the maximum powerpoint for sources 201 a, 201 c and 201 d. Consequently, the arrangementmay be not operated at best achievable efficiency.

Turning back to the example of a solar power system 10 of FIG. 1, fixinga predetermined constant output voltage from the strings 103 may causethe solar panels to supply lower output power than otherwise possible.Further, each string carries a single current that may be passed throughall of the solar panels along the string. If the solar panels aremismatched due to manufacturing differences, aging or if theymalfunction or are placed under different shading conditions, thecurrent, voltage and power output of each panel may be be different.Forcing a single current through all of the panels of the string causesthe individual panels to work at a non-optimal power point and can alsocause panels which may be highly mismatched to generate “hot spots” dueto the high current flowing through them. Due to these and otherdrawbacks of conventional centralized methods, the solar panels have tobe matched properly. In some cases external diodes may be used to bypassthe panels that may be highly mismatched. In conventional multiplestring configurations all strings have to be composed of exactly thesame number of solar panels and the panels may be selected of the samemodel and must be installed at exactly the same spatial orientation,being exposed to the same sunlight conditions at all times. This may bedifficult to achieve and can be very costly.

Various different topologies have been proposed in order to overcome theabove deficiencies of the serial installation. For example, some haveproposed to have inverters coupled to each DC source, and connect all ofthe inverters in parallel. Others have proposed to have DC/DC converterconnected to each DC source, and to connect all of the convertersserially or in parallel to a central inverter. Among the DC/DCconverters proposed for use with the DC sources may be boost converter,buck converter, buck-boost converter, or a Cuk converter. It has alsobeen proposed to incorporate MPPT into each DC power source, e.g., intoeach solar panel, and connect the panels serially.

As already mentioned above, various environmental and operationalconditions impact the power output of DC power sources. In the case ofsolar panels, solar radiance, ambient temperature, and shading, whetherfrom near objects such as trees or far objects such as clouds, impactthe power extracted from each solar panel. Depending on the number andtype of panels used, the extracted power may vary widely in the voltageand current. Owners and even professional installers find it difficultto verify the correct operation of the solar system. With time, manyother factors, such as aging, dust and dirt collection and moduledegradation affect the performance of the solar array.

The sensitivity of photovoltaic panels to external conditions may beeven more profound when concentrated photovoltaics (CPV) may be used. Insuch installations, the sun radiation may be concentrated by use oflenses or mirrors onto small cells. These cells may be much moreefficient then typical PV cells and use a technology knows as double- ortriple-junction, in which a number of p-n junctions may be constructedone on top of the other—each junction converts light from a certain partof the spectrum and allows the rest to pass-through to the nextjunction. Thus, these cells may be much more efficient (with peakefficiencies of over 40%). Since these cells may be expensive, they maybe usually used in CPV applications which call for smaller cells.However, the power output of CPV installations now depends uponfluctuations in the intensity of different parts of the spectrum of thesun (and not only the total intensity), and imperfections or distortionsin the lenses or mirrors used. Thus, having a single MPPT for manypanels will lead to significant power loss, and great benefits may berealized from using a panel- (or cell-)level MPPT as described inaspects of the present invention.

Another field in which traditional photovoltaic installations face manyproblems may be the developing market of building-integratedphotovoltaics (BIPV). In BIPV installations, the panels may beintegrated into buildings during construction—either as roof panels oras structural or additional elements in the walls and windows. Thus,BIPV installations suffer greatly from local partial shading due to theexistence of other structural elements in the vicinity of the panels.Moreover, the panels may be naturally positioned on many differentfacets of the building, and therefore the lighting conditions each panelexperiences may vary greatly. Since in traditional solutions the panelsmay be stringed together to a joint MPPT, much power may be lost. Asolution that could harvest more power would obviously be verybeneficial in installations of this type.

Yet another problem with traditional installations may be the poorenergy utilization in cases of low sun-light. Most inverters require acertain minimal voltage (typically between 150V to 350V) in order tostart functioning. If there may be low light, the aggregated voltagefrom the panels may not reach this minimal value, and the power may bethus lost. A solution that could boost the voltage of panels sufferingfrom low light, would therefore allow for the produced energy to beharvested.

During installation of a solar array according to the conventionalconfigurations 10, the installer can verify the correctness of theinstallation and performance of the solar array by using test equipmentto check the current-voltage characteristics of each panel, each stringand the entire array. In practice, however, individual panels andstrings may be generally either not tested at all or tested only priorto connection. This happens because current measurement may be done byeither a series connection to the solar array or a series resistor inthe array which may be typically not convenient. Instead, onlyhigh-level pass/fail testing of the overall installation may beperformed.

After the initial testing of the installation, the solar array may beconnected to inverter 104 which optionally includes a monitoring modulewhich monitors performance of the entire array. The performanceinformation gathered from monitoring within the inverter 104 includesintegrated power output of the array and the power production rate, butthe information lacks any fine details about the functioning ofindividual solar panels. Therefore, the performance information providedby monitoring at the inverter 104 may be usually not sufficient tounderstand if power loss may be due to environmental conditions, frommalfunctions or from poor installation or maintenance of the solararray. Furthermore, integrated information does not pinpoint which ofsolar panels 101 may be responsible for a detected power loss.

The topology provided by aspects of the present invention may solve manyof the problems associated with, and may have many advantages over, theconventional art topologies. For example, aspects may enable seriallyconnecting mismatched power sources, such as mismatched solar panels,panel of different models and power ratings, and even panels fromdifferent manufacturers and semiconductor materials. It may also allowserial connection of sources operating under different conditions, suchas, e.g., solar panels exposed to different light or temperatureconditions. It may also enable installations of serially connectedpanels at different orientations or different sections of the roof orstructure. This and other features and advantages will become apparentfrom the following detailed description.

Aspects of the present invention provide a system and method forcombining power from multiple DC power sources into a single powersupply. According to aspects of the present invention, each DC powersource may be associated with a DC-DC power converter. Modules formed bycoupling the DC power sources to their associated converters may becoupled in series to provide a string of modules. The string of modulesmay be then coupled to an inverter having its input voltage fixed. Amaximum power point control loop in each converter harvests the maximumpower from each DC power source and transfers this power as output fromthe power converter. For each converter, substantially all the inputpower may be converted to the output power, such that the conversionefficiency may be 90% or higher in some situations. Further, thecontrolling may be performed by fixing the input current or inputvoltage of the converter to the maximum power point and allowing outputvoltage of the converter to vary. For each power source, one or moresensors perform the monitoring of the input power level to theassociated converter. In some aspects of the invention, amicrocontroller may perform the maximum power point tracking and controlin each converter by using pulse width modulation to adjust the dutycycle used for transferring power from the input to the output.

One aspect of the present invention provides a greater degree of faulttolerance, maintenance and serviceability by monitoring, logging and/orcommunicating the performance of each solar panel. In one aspect of theinvention, the microcontroller that may be used for maximum power pointtracking may also be used to perform the monitoring, logging andcommunication functions. These functions allow for quick and easytroubleshooting during installation, thereby significantly reducinginstallation time. These functions may be also beneficial for quickdetection of problems during maintenance work. Aspects of the presentinvention allow easy location, repair, or replacement of failed solarpanels. When repair or replacement may be not feasible, bypass featuresof the current invention provide increased reliability.

In one aspect, the present invention relates to arrays of solar cellswhere the power from the cells may be combined. Each converter may beattached to a single solar cell, or a plurality of cell connected inseries, in parallel, or both, e.g., parallel connection of strings ofserially connected cells. In one feature each converter may be attachedto one panel of photovoltaic strings. However, while applicable in thecontext of solar power technology, the aspects of the present inventionmay be used in any distributed power network using DC power sources. Forexample, they may be used in batteries with numerous cells or hybridvehicles with multiple fuel cells on board. The DC power sources may besolar cells, solar panels, electrical fuel cells, electrical batteries,and the like. Further, although the discussion below relates tocombining power from an array of DC power sources into a source of ACvoltage, the aspects of the present invention may also apply tocombining power from DC sources into another DC voltage.

FIG. 3 illustrates a distributed power harvesting configuration 30,according to an feature of the present invention. Configuration 30enables connection of multiple power sources, for example solar panels301 a-301 d, to a single power supply. In one aspect of the invention,the series string of all of the solar panels may be coupled to aninverter 304. In another aspect of the invention, several seriallyconnected strings of solar panels may be connected to a single inverter304. The inverter 304 may be replaced by other elements, such as, e.g.,a charging regulator for charging a battery bank.

In configuration 30, each solar panel 301 a-301 d may be connected to aseparate power converter circuit 305 a-305 d. One solar panel togetherwith its associated power converter circuit forms a module, e.g., module320. Each converter 305 a-305 d may adapt optimally to the powercharacteristics of the connected solar panel 301 a-301 d and transfersthe power efficiently from converter input to converter output. Theconverters 305 a-305 d can be buck converters, boost converters,buck/boost converters, flyback or forward converters, etc. Theconverters 305 a-305 d may also contain a number of componentconverters, for example a serial connection of a buck and a boostconverter. Each converter 305 a-305 d may include a control loop thatreceives a feedback signal, not from the converter's output current orvoltage, but rather from the converter's input coming from the solarpanel 301. An example of such a control loop may be a maximum powerpoint tracking (MPPT) loop. The MPPT loop in the converter locks theinput voltage and current from each solar panel 301 a-301 d to itsoptimal power point. Conventional DC-to-DC converters may have a wideinput voltage range at their input and an output voltage that may bepredetermined and fixed. In these conventional DC-to-DC voltageconverters, a controller within the converter monitors the current orvoltage at the input, and the voltage at the output. The controllerdetermines the appropriate pulse width modulation (PWM) duty cycle tofix the output voltage to the predetermined value by increasing the dutycycle if the output voltage drops. Accordingly, the conventionalconverter includes a feedback loop that closes on the output voltage anduses the output voltage to further adjust and fine tune the outputvoltage from the converter. As a result of changing the output voltage,the current extracted from the input may be also varied. In theconverters 305 a-305 d, according to aspects of the present invention, acontroller within the converter 405 monitors the voltage and current atthe converter input and determines the PWM in such a way that maximumpower may be extracted from the attached panel 301 a-301 d. Thecontroller of the converter 405 dynamically tracks the maximum powerpoint at the converter input. In the aspects of the present invention,the feedback loop may be closed on the input power in order to trackmaximum input power rather than closing the feedback loop on the outputvoltage as performed by conventional DC-to-DC voltage converters.

As a result of having a separate MPPT circuit in each converter 305a-305 d, and consequently for each solar panel 301 a-301 d, each string303 in the feature shown in FIG. 3 may have a different number ordifferent brand of panels 301 a-301 d connected in series. The circuitof FIG. 3 continuously performs MPPT on the output of each solar panel301 a-301 d to react to changes in temperature, solar radiance, shadingor other performance factors that impact that particular solar panel 301a-301 d. As a result, the MPPT circuit within the converters 305 a-305 dmay harvest the maximum possible power from each panel 301 a-301 d andtransfers this power as output regardless of the parameters impactingthe other solar panels.

As such, the aspects of the invention shown in FIG. 3 continuously trackand maintain the input current and the input voltage to each converterat the maximum power point of the DC power source providing the inputcurrent and the input voltage to the converter. The maximum power of theDC power source that may be input to the converter may be also outputfrom the converter. The converter output power may be at a current andvoltage different from the converter input current and voltage. Theoutput current and voltage from the converter may be responsive torequirements of the series connected portion of the circuit.

In one aspect of the invention, the outputs of converters 305 a-305 dmay be series connected into a single DC output that forms the input tothe load or power supplier, in this example, inverter 304. The inverter304 converts the series connected DC output of the converters into an ACpower supply. The load, in this case inverter 304, regulates the voltageat the load's input. That may be, in this example, an independentcontrol loop 320 holds the input voltage at a set value, say 400 volts.Consequently, the input current to the inverter may be dictated by theavailable power, and this may be the current that flows through allserially connected DC sources. On the other hand, while the output ofthe DC-DC converters must be at the current input of the inverter, thecurrent and voltage input to the converter may be independentlycontrolled using the MPPT.

In the conventional art, the input voltage to the load was allowed tovary according to the available power. For example, when a lot ofsunshine may be available in a solar installation, the voltage input tothe inverter can vary even up to 1000 volts.

Consequently, as sunshine illumination varies, the voltage varies withit, and the electrical components in the inverter (or other powersupplier or load) may be exposed to varying voltage. This tends todegrade the performance of the components and ultimately causes them tofail. On the other hand, by fixing the voltage or current to the inputof the load or power supplier, here the inverter, the electricalcomponents may be always exposed to the same voltage or current andtherefore would have extended service life. For example, the componentsof the load (e.g., capacitors, switches and coil of the inverter) may beselected so that at the fixed input voltage or current they operate at,say, 60% of their rating. This would improve the reliability and prolongthe service life of the component, which may be critical for avoidingloss of service in applications such as solar power systems.

FIGS. 4A and 4B illustrate the operation of the system of FIG. 3 underdifferent conditions, according to aspects of the invention. Theexemplary configuration 40 may be similar to configuration 30 of FIG. 3.In the example shown, ten DC power sources 401/1 through 401/10 may beconnected to ten power converters 405/1 through 405/10, respectively.The modules formed by the DC power sources and their correspondingconverters may be coupled together in series to form a string 403. Inone aspect of the invention, the series-connected converters 405 may becoupled to a DC-to-AC inverter 404.

The DC power sources may be solar panels and the example may bediscussed with respect to solar panels as one illustrative case. Eachsolar panel 401 may have a different power output due to manufacturingtolerances, shading, or other factors. For the purpose of the presentexample, an ideal case may be illustrated in FIG. 4A, where efficiencyof the DC-to-DC conversion may be assumed to be 100% and the panels 501may be assumed to be identical. In some aspects of the invention,efficiencies of the converters may be quite high and range at about95%-99%. So, the assumption of 100% efficiency may be not unreasonablefor illustration purposes. Moreover, according to embodiments of thesubject invention, each of the DC-DC converters may be constructed as apower converter, i.e., it transfers to its output the entire power itreceives in its input with very low losses.

Power output of each solar panel 401 may be maintained at the maximumpower point for the panel by a control loop within the correspondingpower converter 405. In the example shown in FIG. 4A, all of the panelsmay be exposed to full sun illumination and each solar panel 401provides 200 W of power. Consequently, the MPPT loop will draw currentand voltage level that will transfer the entire 200 W from the panel toits associated converter. That may be, the current and voltage dictatedby the MPPT form the input current I_(in) and input voltage V_(in) tothe converter. The output voltage may be dictated by the constantvoltage set at the inverter 404, as will be explained below. The outputcurrent I_(out) would then be the total power, i.e., 200 W, divided bythe output voltage V_(out).

As noted above, according to a feature of the invention, the inputvoltage to inverter 404 may be controlled by the inverter (in thisexample, kept constant), by way of control loop 420. For the purpose ofthis example, assume the input voltage may be kept as 400V (which may bean ideal value for inverting to 220VAC). Since we assume that there maybe ten serially connected power converters, each providing 200 W, we cansee that the input current to the inverter 404 may be 2000 W/400V=5 A.Thus, the current flowing through each of the converters 401/1-401/10must be 5 A. This means that in this idealized example each of theconverters provides an output voltage of 200 W/5 A=40 V. Now, assumethat the MPPT for each panel (assuming perfect matching panels) dictatesV_(MPP)==32V. This means that the input voltage to the inverter would be32V, and the input current would be 200 W/32V=6.25 A.

We now turn to another example, wherein the system may be stillmaintained at an ideal mode (i.e., perfectly matching DC sources andentire power may be transferred to the inverter), but the environmentalconditions may be not ideal. For example, one DC source may beoverheating, may be malfunctioning, or, as in the example of FIG. 4B,the ninth solar panel 401/9 may be shaded and consequently produces only40 W of power. Since we keep all other conditions as in the example ofFTG. 4A, the other nine solar panels 401 may be unshaded and stillproduce 200 W of power. The power converter 405/9 includes MPPT tomaintain the solar panel 501/9 operating at the maximum power point,which may be now lowered due to the shading.

The total power available from the string may be now 9×200 W+40 W=1840W. Since the input to the inverter may be still maintained at 400V, theinput current to the inverter will now be 1840 W/40V=4.6 A. This meansthat the output of all of the power converters 405/1-405/10 in thestring must be at 4.6 A. Therefore, for the nine unshaded panels, theconverters will output 200 W/4.6 A=43.5V. On the other hand, theconverter 405/9 attached to the shaded panel 401/9 will output 40 W/4.6A=8.7V. Checking the math, the input to the inverter can be obtained byadding nine converters providing 43.5V and one converter providing 8.7V,i.e., (9×43.5V)+8.7V=400V.

The output of the nine non-shaded panels would still be controlled bythe MPPT as in FIG. 4A, thereby standing at 32V and 6.25 A. On the otherhand, since the nines panel 401/9 may be shaded, lets assume its MPPTdropped to 28V. Consequently, the output current of the ninth panel is40 W/28V=1.43 A. As can be seen by this example, all of the panels maybe operated at their maximum power point, regardless of operatingconditions. As shown by the example of FIG. 4B, even if the output ofone DC source drops dramatically, the system still maintains relativelyhigh power output by fixing the voltage input to the inverter, andcontrolling the input to the converters independently so as to drawpower from the DC source at the MPP.

As can be appreciated, the benefit of the topology illustrated in FIGS.4A and 4B may be numerous. For example, the output characteristics ofthe serially connected DC sources, such as solar panels, need not match.Consequently, the serial string may utilize panels from differentmanufacturers or panels installed on different parts of the roofs (i.e.,at different spatial orientation). Moreover, if several strings areconnected in parallel, it may be not necessary that the strings match;rather each string may have different panels or different number ofpanels. This topology may also enhance reliability by alleviating thehot spot problem. That may be, as shown in FIG. 4A the output of theshaded panel 401/9 may be 1.43 A, while the current at the output of theun-shaded panels may be 6.25 A. This discrepancy in current when thecomponents may be series connected causes a large current being forcedthrough the shaded panel that may cause overheating and malfunction atthis component. However, by aspects of the topology described herein,the input voltage may be set independently, and the power draw from eachpanel to its converter may be set independently according to the panelsMPP at each point in time, the current at each panel may be independenton the current draw from the serially connected converters.

It may be easily realized that since the power may be optimizedindependently for each panel, panels could be installed in differentfacets and directions in BIPV installations. Thus, the problem of lowpower utilization in building-integrated installations may be solved,and more installations may now be profitable.

The aspects of described system may also solve the problem of energyharvesting in low light conditions. Even small amounts of light may beenough to make the converters 405 operational, and they then starttransferring power to the inverter. If small amounts of power may beavailable, there will be a low current flow but the voltage will be highenough for the inverter to function, and the power will indeed beharvested.

According to aspects of the invention, the inverter 404 includes acontrol loop 420 to maintain an optimal voltage at the input of inverter404. In the example of FIG. 4B, the input voltage to inverter 404 may bemaintained at 400V by the control loop 420. The converters 405 may betransferring substantially all of the available power from the solarpanels to the input of the inverter 404. As a result, the input currentto the inverter 404 may be dependent only on the power provided by thesolar panels and the regulated set, i.e., constant, voltage at theinverter input.

The conventional inverter 104, shown in FIG. 1 and FIG. 3A, may berequired to have a very wide input voltage to accommodate for changingconditions, for example a change in luminance, temperature and aging ofthe solar array. This may be in contrast to the inverter 404 that may bedesigned according to aspects of the present invention. The inverter 404does not require a wide input voltage and may be therefore simpler todesign and more reliable. This higher reliability may be achieved, amongother factors, by the fact that there may be no voltage spikes at theinput to the inverter and thus the components of the inverter experiencelower electrical stress and may last longer.

When the inverter 404 is a part of the circuit, the power from thepanels may be transferred to a load that may be connected to theinverter. To enable the inverter 404 to work at its optimal inputvoltage, any excess power produced by the solar array, and not used bythe load, may be dissipated. Excess power may be handled by selling theexcess power to the utility company if such an option may be available.For off-grid solar arrays, the excess power may be stored in batteries.Yet another option may be to connect a number of adjacent housestogether to form a micro-grid and to allow load-balancing of powerbetween the houses. If the excess power available from the solar arraymay be not stored or sold, then another mechanism may be provided todissipate excess power.

The features and benefits explained with respect to FIGS. 4A and 4Bstem, at least partially, from having the inverter dictate the voltageprovided at its input. Conversely, a design can be implemented whereinthe inverter dictates the current at its input. Such an arrangement isillustrated in FIG. 4C. FIG. 4C illustrates an feature of the inventionwherein the inverter controls the input current. Power output of eachsolar panel 401 may be maintained at the maximum power point for thepanel by a control loop within the corresponding power converter 405. Inthe example shown in FIG. 4C, all of the panels may be exposed to fullsun illumination and each solar panel 401 provides 200 W of power.Consequently, the MPPT loop will draw current and voltage level thatwill transfer the entire 200 W from the panel to its associatedconverter. That may be, the current and voltage dictated by the MPPTform the input current I_(in) and input voltage V_(in) to the converter.The output voltage may be dictated by the constant current set at theinverter 404, as will be explained below. The output voltage V_(out)would then be the total power, i.e., 200 W, divided by the outputcurrent I_(out).

As noted above, according to a feature of the invention, the inputcurrent to inverter 404 may be dictated by the inverter by way ofcontrol loop 420. For the purpose of this example, assume the inputcurrent may be kept as 5 A. Since we assume that there may be tenserially connected power converters, each providing 200 W, we can seethat the input voltage to the inverter 404 is 2000 W/5 A=400V. Thus, thecurrent flowing through each of the converters 401/1-401/10 must be 5 A.This means that in this idealized example each of the convertersprovides an output voltage of 200 W/5 A=40V. Now, assume that the MPPTfor each panel (assuming perfect matching panels) dictates V_(MPP)=32V.This means that the input voltage to the inverter would be 32V, and theinput current would be 200 W/32V=6.25 A.

Consequently, similar advantages may be achieved by having the invertercontrol the current, rather than the voltage. However, unlike theconventional art, changes in the output of the panels may not causechanges in the current flowing to the inverter, as that may be dictatedby the inverter itself. Therefore, if the inverter may be designed tokeep the current or the voltage constant, then regardless of theoperation of the panels, the current or voltage to the inverter willremain constant.

FIG. 5 illustrates a distributed power harvesting system, according toother aspects of the invention, using DC power sources. FIG. 5illustrates multiple strings 503 coupled together in parallel. Each ofthe strings may be a series connection of multiple modules and each ofthe modules includes a DC power source 501 that may be coupled to aconverter 505. The DC power source may be a solar panel. The output ofthe parallel connection of the strings 503 may be connected, again inparallel, to a shunt regulator 506 and a load controller 504. The loadcontroller 504 may be an inverter as with the embodiments of FIGS. 4Aand 4B. Shunt regulators automatically maintain a constant voltageacross its terminals. The shunt regulator 506 may be configured todissipate excess power to maintain the input voltage at the input to theinverter 504 at a regulated level and prevent the inverter input voltagefrom increasing. The current which flows through shunt regulator 506 maycomplement the current drawn by inverter 504 in order to ensure that theinput voltage of the inverter may be maintained at a constant level, forexample at 400V.

By fixing the inverter input voltage, the inverter input current may bevaried according to the available power draw. This current may bedivided between the strings 503 of the series connected converters. Wheneach converter includes a controller loop maintaining the converterinput voltage at the maximum power point of the associated DC powersource, the output power of the converter may be determined. Theconverter power and the converter output current together determine theconverter output voltage. The converter output voltage may be used by apower conversion circuit in the converter for stepping up or steppingdown the converter input voltage to obtain the converter output voltagefrom the input voltage as determined by the MPPT.

FIG. 6 illustrates an exemplary DC-to-DC converter 605 according toaspects of the invention. DC-to-DC converters may be conventionally usedto either step down or step up a varied or constant DC voltage input toa higher or a lower constant voltage output, depending on therequirements of the circuit. However, in the feature of FIG. 6 the DC-DCconverter may be used as a power converter, i.e., transferring the inputpower to output power, the input voltage varying according to the MPPT,while the output current being dictated by the constant input voltage tothe inverter. That may be, the input voltage and current may vary at anytime and the output voltage and current may vary at any time, dependingon the operating condition of the DC power sources.

The converter 605 may be connected to a corresponding DC power source601 at input terminals 614 and 616. The converted power of the DC powersource 601 may be output to the circuit through output terminals 610,612. Between the input terminals 614, 616 and the output terminals 610,612, the remainder of the converter circuit may be located that includesinput and output capacitors 620, 640, back flow prevention diodes 622,642 and a power conversion circuit including a controller 606 and aninductor 608.

The inputs 616 and 614 may be separated by a capacitor 620 which acts asan open to a DC voltage. The outputs 610 and 612 may be also separatedby a capacitor 640 that also acts an open to DC output voltage. Thesecapacitors may be DC-blocking or AC-coupling capacitors that short whenfaced with alternating current of a frequency for which they may beselected. Capacitor 640 coupled between the outputs 610, 612 and alsooperates as a part of the power conversion circuit discussed below.

Diode 642 may be coupled between the outputs 610 and 612 with a polaritysuch that current may not backflow into the converter 605 from thepositive lead of the output 612. Diode 622 may be coupled between thepositive output lead 612 through inductor 608 which acts a short for DCcurrent and the negative input lead 614 with such polarity to prevent acurrent from the output 612 to backflow into the solar panel 601.

The DC power sources 601 may be solar panels. A potential differenceexists between the wires 614 and 616 due to the electron-hole pairsproduced in the solar cells of panel 601. The converter 605 may maintainmaximum power output by extracting current from the solar panel 601 atits peak power point by continuously monitoring the current and voltageprovided by the panel and using a maximum power point trackingalgorithm. The controller 606 may include an MPPT circuit or algorithmfor performing the peak power tracking. Peak power tracking and pulsewidth modulation, PWM, may be performed together to achieve the desiredinput voltage and current. The MPPT in the controller 606 may be anyconventional MPPT, such as, e.g., perturb and observe (P&O), incrementalconductance, etc. However, notably the MPPT may be performed on thepanel directly, i.e., at the input to the converter, rather than at theoutput of the converter. The generated power may be then transferred tothe output terminals 610 and 612. The outputs of multiple converters 605may be connected in series, such that the positive lead 612 of oneconverter 605 may be connected to the negative lead 610 of the nextconverter 605.

In FIG. 6, the converter 605 is shown as a buck plus boost converter.The term “buck plus boost” as used herein is a buck converter directlyfollowed by a boost converter as shown in FIG. 6, which may also appearin the literature as “cascaded buck-boost converter”. If the voltage isto be lowered, the boost portion may be substantially shorted. If thevoltage may is be raised, the buck portion may be substantially shorted.The term “buck plus boost” differs from buck/boost topology which may bea classic topology that may be used when voltage is to be raised orlowered. The efficiency of “buck/boost” topology may be inherently lowerthen a buck or a boost. Additionally, for given requirements, abuck-boost converter may need bigger passive components then a buck plusboost converter in order to function. Therefore, the buck plus boosttopology of FIG. 6 may have a higher efficiency than the buck/boosttopology. However, the circuit of FIG. 6 continuously decides whether itmay be bucking or boosting. In some situations when the desired outputvoltage may be similar to the input voltage, then both the buck andboost portions may be operational.

The controller 606 may include a pulse width modulator, PWM, or adigital pulse width modulator, DPWM, to be used with the buck and boostconverter circuits. The controller 606 controls both the buck converterand the boost converter and determines whether a buck or a boostoperation is to be performed. In some circumstances both the buck andboost portions may operate together. That may be, as explained withrespect to the embodiments of FIGS. 4A and 4B, the input voltage andcurrent may be selected independently of the selection of output currentand voltage. Moreover, the selection of either input or output valuesmay change at any given moment, depending on the operation of the DCpower sources. Therefore, in the feature of FTG. 6 the converter may beconstructed so that at any given time a selected value of input voltageand current may be up converted or down converted depending on theoutput requirement.

In one implementation, an integrated circuit (IC) 604 may be used thatincorporates some of the functionality of converter 605. IC 604 may beoptionally a single ASIC able to withstand harsh temperature extremespresent in outdoor solar installations. ASIC 604 may be designed for ahigh mean time between failures (MTBF) of more than 25 years. However, adiscrete solution using multiple integrated circuits may also be used ina similar manner. In the exemplary feature shown in FIG. 6, the buckplus boost portion of the converter 605 may be implemented as the IC604. Practical considerations may lead to other segmentations of thesystem. For example, in one aspect, the IC 604 may include two ICs, oneanalog IC which handles the high currents and voltages in the system,and one simple low-voltage digital IC which includes the control logic.The analog IC may be implemented using power FETs which mayalternatively be implemented in discrete components, FET drivers, A/Ds,and the like. The digital IC may form the controller 606.

In the exemplary circuit shown, the buck converter includes the inputcapacitor 620, transistors 628 and 630 a diode 622 positioned inparallel to transistor 628, and an inductor 608. The transistors 628,630 each have a parasitic body diode 624, 626. In the exemplary circuitshown, the boost converter includes the inductor 608, which may beshared with the buck converter, transistors 648 and 650 a diode 642positioned in parallel to transistor 650, and the output capacitor 640.The transistors 648, 650 each have a parasitic body diode 644, 646.

As shown in FIG. 1, adding electronic elements in the series arrangementmay reduce the reliability of the system, because if one electricalcomponent breaks it may affect the entire system. Specifically, if afailure in one of the serially connected elements causes an open circuitin the failed element, current ceases to flow through the entire series,thereby causing the entire system to stop function. Aspects of thepresent invention provide a converter circuit where electrical elementsof the circuit have one or more bypass routes ssociated with them thatcarry the current in case of the electrical element fails. For example,each switching transistor of either the buck or the boost portion of theconverter has its own bypass. Upon failure of any of the switchingtransistors, that element of the circuit may be bypassed. Also, uponinductor failure, the current bypasses the failed inductor through theparasitic diodes of the transistor used in the boost converter.

FIG. 7 illustrates a power converter, according to aspects of theinvention. FIG. 7 highlights, among others, a monitoring and controlfunctionality of a DC-to-DC converter 705. A DC voltage source 701 isalso shown in the figure. Portions of a simplified buck and boostconverter circuit may be shown for the converter 705. The portions showninclude the switching transistors 728, 730, 748 and 750 and the commoninductor 708. Each of the switching transistors may be controlled by apower conversion controller 706. The power conversion controller 706includes the pulse-width modulation (PWM) circuit 733, and a digitalcontrol machine 730 including a protection portion 737. The powerconversion controller 706 may be coupled to microcontroller 790, whichincludes an MPPT module 719, and may also optionally include acommunication module 709, a monitoring and logging module 711, and aprotection module 735.

A current sensor 703 may be coupled between the DC power source 701 andthe converter 705, and output of the current sensor 703 may be providedto the digital control machine 730 through an associated analog todigital converter 723. A voltage sensor 704 may be coupled between theDC power source 701 and the converter 705 and output of the voltagesensor 704 may be provided to the digital control machine 730 through anassociated analog to digital converter 724. The current sensor 703 andthe voltage sensor 704 may be used to monitor current and voltage outputfrom the DC power source, e.g., the solar panel 701. The measuredcurrent and voltage may be provided to the digital control machine 730and may be used to maintain the converter input power at the maximumpower point.

The PWM circuit 733 controls the switching transistors of the buck andboost portions of the converter circuit. The PWM circuit may be adigital pulse-width modulation (DPWM) circuit. Outputs of the converter705 taken at the inductor 708 and at the switching transistor 750 may beprovided to the digital control machine 730 through analog to digitalconverters 741, 742, so as to control the PWM circuit 733.

A random access memory (RAM) module 715 and a non-volatile random accessmemory (NVRAM) module 713 may be located outside the microcontroller 790but coupled to the microcontroller 790. A temperature sensor 779 and oneor more external sensor interfaces 707 may be coupled to themicrocontroller 790. The temperature sensor 779 may be used to measurethe temperature of the DC power source 701. A physical interface 717 maybe coupled to the microcontroller 790 and used to convert data from themicrocontroller into a standard communication protocol and physicallayer. An internal power supply unit 739 may be included in theconverter 705.

In various aspects, the current sensor 703 may be implemented by varioustechniques used to measure current. In one aspect, the currentmeasurement module 703 may be implemented using a very low valueresistor. The voltage across the resistor will be proportional to thecurrent flowing through the resistor. In another aspect, the currentmeasurement module 703 may be implemented using current probes which usethe Hall Effect to measure the current through a conductor withoutadding a series resistor. After translating the current to voltage, thedata may be passed through a low pass filter and then digitized. Theanalog to digital converter associated with the current sensor 703 isshown as the A/D converter 723 in FIG. 7. Aliasing effect in theresulting digital data may be avoided by selecting an appropriateresolution and sample rate for the analog to digital converter. If thecurrent sensing technique does not require a series connection, then thecurrent sensor 703 may be connected to the DC power source 701 inparallel.

In one aspect, the voltage sensor 704 uses simple parallel voltagemeasurement techniques in order to measure the voltage output of thesolar panel. The analog voltage may be passed through a low pass filterin order to minimize aliasing. The data may be then digitized using ananalog to digital converter. The analog to digital converter associatedwith the voltage sensor 704 may be shown as the A/D converter 724 inFIG. 7. The A/D converter 724 has sufficient resolution to generate anadequately sampled digital signal from the analog voltage measured atthe DC power source 701 that may be a solar panel.

The current and voltage data collected for tracking the maximum powerpoint at the converter input may be used for monitoring purposes also.An analog to digital converter with sufficient resolution may correctlyevaluate the panel voltage and current. However, to evaluate the stateof the panel, even low sample rates may be sufficient. A low-pass filtermakes it possible for low sample rates to be sufficient for evaluatingthe state of the panel. The current and voltage date may be provided tothe monitoring and logging module 711 for analysis.

The temperature sensor 779 enables the system to use temperature data inthe analysis process. The temperature may be indicative of some types offailures and problems. Furthermore, in the case that the power sourcemay be a solar panel, the panel temperature may be a factor in poweroutput production.

The one or more optional external sensor interfaces 707 enableconnecting various external sensors to the converter 705. Externalsensors may be used to enhance analysis of the state of the solar panel701, or a string or an array formed by connecting the solar panels 701.Examples of external sensors include ambient temperature sensors, solarradiance sensors, and sensors from neighboring panels. External sensorsmay be integrated into the converter 705 instead of being attachedexternally.

In one aspect, the information acquired from the current and voltagesensors 703, 704 and the optional temperature and external sensors 705,707 may be transmitted to a central analysis station for monitoring,control, and analysis using the communications interface 709. Thecentral analysis station is not shown in the figure. The communicationinterface 709 connects a microcontroller 790 to a communication bus. Thecommunication bus can be implemented in several ways. In one aspect, thecommunication bus may be implemented using an off-the-shelfcommunication bus such as Ethernet or RS422. Other methods such aswireless communications or power line communications, which could beimplemented on the power line connecting the panels, may also be used.If bidirectional communication is used, the central analysis station mayrequest the data collected by the microcontroller 790. Alternatively orin addition, the information acquired from sensors 703, 704, 705, 707may be logged locally using the monitoring and logging module 711 inlocal memory such as the RAM 715 or the NVRAM 713.

Analysis of the information from sensors 703, 704, 705, 707 may enabledetection and location of many types of failures associated with powerloss in solar arrays. Smart analysis may also be used to suggestcorrective measures such as cleaning or replacing a specific portion ofthe solar array. Analysis of sensor information may also detect powerlosses caused by environmental conditions or installation mistakes andprevent costly and difficult solar array testing.

Consequently, in one aspect, the microcontroller 790 simultaneouslymaintains the maximum power point of input power to the converter 705from the attached DC power source or solar panel 701 based on the MPPTalgorithm in the MPPT module 719 and manages the process of gatheringthe information from sensors 703, 704, 705, 707. The collectedinformation may be stored in the local memory 713, 715 and transmittedto an external central analysis station. In one aspect, themicrocontroller 790 uses previously defined parameters stored in theNVRAM 713 in order to operate. The information stored in the NVRAM 713may include information about the converter 705 such as serial number,the type of communication bus used, the status update rate and the ID ofthe central analysis station. This information may be added to theparameters collected by the sensors before transmission.

The converters 705 may be installed during the installation of the solararray or retrofitted to existing installations. In both cases, theconverters 705 may be connected to a panel junction connection box or tocables connecting the panels 701. Each converter 705 may be providedwith the connectors and cabling to enable easy installation andconnection to solar panels 701 and panel cables.

In one aspect, the physical interface 717 may be used to convert to astandard communication protocol and physical layer so that duringinstallation and maintenance, the converter 705 may be connected to oneof various data terminals, such as a computer or PDA. Analysis may thenbe implemented as software which will be run on a standard computer, anembedded platform or a proprietary device.

The installation process of the converters 705 includes connecting eachconverter 705 to a solar panel 701. One or more of the sensors 703, 704,705, 707 may be used to ensure that the solar panel 701 and theconverter 705 may be properly coupled together. During installation,parameters such as serial number, physical location and the arrayconnection topology may be stored in the NVRAM 713. These parameters maybe used by analysis software to detect future problems in solar panels701 and arrays.

When the DC power sources 701 are solar panels, one of the problemsfacing installers of photovoltaic solar panel arrays may be safety. Thesolar panels 701 may be connected in series during the day when theremay be sunlight. Therefore, at the final stages of installation, whenseveral solar panels 701 are connected in series, the voltage across astring of panels may reach dangerous levels. Voltages as high as 600Vmay be common in domestic installations. Thus, the installer faces adanger of electrocution. The converters 705 that are connected to thepanels 701 may use built-in functionality to prevent such a danger. Forexample, the converters 705 may include circuitry or hardware ofsoftware safety module that limits the output voltage to a safe leveluntil a predetermined minimum load may be detected. Only after detectingthis predetermined load, the microcontroller 790 ramps up the outputvoltage from the converter 705.

Another method of providing a safety mechanism may be to usecommunications between the converters 705 and the associated inverterfor the string or array of panels. This communication, that may be forexample a power line communication, may provide a handshake before anysignificant or potentially dangerous power level may be made available.Thus, the converters 705 would wait for an analog or digital releasesignal from the inverter in the associated array before transferringpower to inverter.

The above methodology for monitoring, control and analysis of the DCpower sources 701 may be implemented on solar panels or on strings orarrays of solar panels or for other power sources such as batteries andfuel cells.

Most photovoltaic panels (mono-crystalline, polycrystalline) comprise ofa series connection of photodiodes. Such a connection may be sensitiveto the hot spot phenomena. In cases where one of the cells in the stringmay be not able to provide the current drawn from the panel, and may gointo reverse bias heating up and finally burning the panel. Panelmanufacturers have found a solution to this problem by splitting theseries of photodiodes into several sub-strings. Each sub-string has itsown bypass diode which enables the panel current to bypass a sub-stringwhich has cells which would otherwise be in reverse bias. The bypassdiode prevents the panel from burning up but prevents the specificsub-string from producing any power at all. In order to achieve themaximum power from the panel each sub-string has to be at maximum powerpoint (MPP) without regard to the other sub-strings. Monocrystallinesolar panels suffer a reduction in output once the temperature from thesunlight reaches around fifty degrees Celsius/a hundred and fifteendegrees Fahrenheit. Reductions of between twelve and fifteen percent canbe expected. These may be lower than the reductions in outputexperienced by polycrystalline cells, but they still need to be factoredinto the calculations and design for any solar power system.

FIG. 8 illustrates characteristic current-voltage curves of a singlephotovoltaic cell at different illumination levels. Curve 800 shows themaximum power point (MPP) for low light levels, curve 802 show themaximum power point MPP for higher light levels, and curve 804 shows themaximum power point MPP yet higher light levels assuming a constanttemperature of the photovoltaic cell. As can be seen, at the differentlight levels the maximum power point may be achieved at nearly identicalvoltages, but at different currents depending on the incident solarirradiance.

FIG. 9 illustrates an arrangement for internal connections of a solarpanel 301 according to conventional art. In FIG. 9, solar panel 301includes photovoltaic cells 905, which may be grouped intoserially-connected strings 910. Strings 910 may be connected together inseries. For each string 910, a bypass diode 920 may be provided so thatin the event of drop in power output of one string 910, that string 910may be bypassed via respective bypass diode 920 instead of having thecells enter a negative voltage region, which will lead to powerdissipation across them and may cause them to burn out. Bypass diodes920 may be connected in parallel across sub-strings 910 for instanceaccording to IEC61730-2 solar safety standards. Bypass diodes 920 mayalso provide a current path around sub-string 910 during dark orpartially shaded conditions. The current path may also allow current toflow through bypass diode 920 in the forward mode, preventing commonthermal failures in string 910 such as cell breakdown or hot spots.During forward mode, bypass diode 920 may have low forward resistance toreduce the wasted power. During normal operation when all strings 910may be irradiated, forward current will flow through strings 910 whilebypass diodes 920 will operate in the reverse blocking mode. In reverseblocking mode, it may be important that bypass diodes 920 have lowhigh-temperature reverse leakage current (I_(R)) to achieve the highpower generation efficiency for each sub-string 910. However, whencurrent flows through bypass diodes 920 they dissipate energy. Forexample, if a current of 5 ampere (A) flows through bypass diode havinga typically 0.7 volt forward bias voltage, the loss through bypass diode920 may be 3.5 W. In practice the loss may easily amount to 10 W.

Reference is now made to FIG. 10 which shows a solar panel 301Paccording to an aspect of the present invention. In FIG. 10, solar panel301P includes photovoltaic cells 905 interconnected together in seriesto form sub-strings 910. Two output nodes X and Y of serially-connectedsub-strings 910 may be connected in connection box 1008 to bus-bar 1010and bus-bar 1012 respectively. The connection of the two output nodes Xand Y of strings 910 to bus-bar 1010 and bus-bar 1012 connects strings910 in parallel giving an output 1016. Output 1016 may be applied acrossbypass diode 920 and to the input of converter 305 with control loop 320which may maintain maximum power at input to converter 305 by settingthe input voltage to converter 305 or by setting the input current toconverter 305. Alternatively output 1016 can be connected to a load suchas an inverter, a conventional direct-current to direct-current (DC-DC)converter or a bank of batteries. Typically bypass diode 920 bypassescurrent around strings 910 when there may be a failure in any of strings910 or when there may be a partial shading of strings 910.

Reference is now also made to FIG. 11 showing a method 1100 accordingvarious exemplary aspects. The outputs of sub strings 910 may beconnected together in parallel via bus-bar 1010 and bus-bar 1012 atnodes X and Y (step 1102). A single bypass diode 920 may be connected inparallel across the input of power converter 305 (step 1104). The inputof power converter 305 may be connected to bus-bar 1010 and bus-bar 1012at nodes V and W respectively (step 1106). Power converter 305, convertsthe power from parallel connected sub-strings 910 to an output power atthe output of power converter 305 (step 1108). The input power to powerconverter 305 may be determined by previously determined criterion andimplemented using control loop 320. In the case of parallel connectedsubstrings 920, the previously determined criterion may provideincreased voltage output and reduced current output of power converter305. The output terminals of power converter 305 may be then connectedto a load (step 1110). The load may be typically a power grid or a bankof batteries. Typically the conversion of power from sub-strings 910 bypower by power converter 305 produces an output power with reducedcurrent (step 1112). The reduced current (step 1112) from the output ofpower converter 305 produces decreased energy losses (proportional tocurrent squared) in the cables connecting the output of converter 305 tothe load.

During operation of panel 301P , by virtue of the additive property ofcurrents in parallel connections, the maximum power point (MPP) trackingof parallel-connected sub-strings 910 may also maximize currents flowingindividually through parallel-connected sub-strings 910. Strings 910contribute to the total current of panel 301P according to their inputpower/lighting condition. In the case where panel 301P may be made froma mono-crystalline/polycrystalline structure; the photodiode voltage ofmono-crystalline/polycrystalline cells may change very slightly withlighting condition. A major factor for cell voltage may be temperaturewhich may change only slightly in a single photovoltaic panel. Solarpanel 301P requires a single bypass diode 920 while solar panel 301 inFIG. 9 requires multiple bypass diodes 920 one for each string 910.During normal operation of panel 301P when all strings 910 may beabsorbing light, bypass diode 920 passes one high temperature reverseleakage current (I_(R)) as opposed to more than one high temperaturereverse leakage current (I_(R)) as shown with solar panel 301 shown inFIG. 9. The voltage of the parallel connected outputs of strings 910 ofpanel 301P in FIG. 10 may be lower than voltage output of panel 301shown in FIG. 9 but the combined current output of panel 301P in FIG. 10may be greater than the current output of panel 301 in FIG. 9 for equalnumbers of strings 910.

Connection of converter 305 and adjustment of control loop 320 in panel301P (FIG. 10) allows for reduced power loss in the conductors(proportional to current squared) of a load attached to the output ofconverter 305 whilst maintaining maximum power transfer to the load.Typically if string 910 has an output voltage V_(S) and current I_(S)for example. Three strings 910 connected in series as in panel 301 (FIG.9) produces an output voltage 3V_(S) and current I_(S). Three stringsconnected in parallel as in FIG. 10 produces an output voltage V_(S) andcurrent 3I_(S). Connection of converter 305 input is across the threestrings 910 connected in parallel in panel 301P in FIG. 10. Theadjustment of control loop 320 may produce an output voltage fromconverter 305 of 3V_(S) and current I_(S); the power loss (currentsquared times cable impedance) in the cables of a load attached to theoutput of converter 305 (whilst converter 305 maintains maximum powertransfer to the load) may be 9 times less, i.e. I_(S) squared as opposedto 3I_(S) squared.

The definite articles “a”, “an” is used herein, such as “a converter”,“an output connector” have the meaning of “one or more” that is “one ormore converters” or “one or more output connectors”.

Although selected aspects have been shown and described, it is to beunderstood the aspects are not limited to the described aspects.Instead, it is to be appreciated that changes may be made to theseaspects without departing from the principles and spirit of theinvention, the scope of which is defined by the claims and theequivalents thereof.

1-17. (canceled)
 18. A system comprising: a string comprising: aplurality of photovoltaic panels; and a plurality of power convertersconnected in series, wherein: each one of the plurality of powerconverters is coupled with a corresponding one of the plurality ofphotovoltaic panels, each one of the plurality of power converters isconfigured to draw power from the corresponding one of the plurality ofphotovoltaic panels according to a corresponding maximum power pointtracking (MPPT) algorithm, and at least two of the plurality ofphotovoltaic panels are building integrated photovoltaic panels.
 19. Thesystem of claim 18, further comprising an inverter coupled to thestring, wherein the inverter is configured to regulate a common voltageacross inputs of the inverter.
 20. The system of claim 18, wherein eachof the plurality of power converters is configured to limit, based on acondition, an output voltage of the power converter to a safe level. 21.The system of claim 20, wherein the condition is one of a predeterminedminimum load, or a signal sent from an inverter.
 22. The system of claim19, further comprising a battery connected to the inverter, wherein theinverter is configured to charge the battery with power provided by thestring.
 23. The system of claim 18, further comprising a second string,the second string comprising: a second plurality of photovoltaic panels;and a second plurality of power converters connected in series, wherein:each one of the second plurality of power converters is coupled with acorresponding one of the second plurality of photovoltaic panels, eachone of the second plurality of power converters is configured to drawpower from the corresponding one of the second plurality of photovoltaicpanels according to a corresponding maximum power point tracking (MPPT)algorithm, and at least two of the second plurality of photovoltaicpanels are building integrated photovoltaic panels.
 24. The system ofclaim 23, wherein a number of the plurality of photovoltaic panels ofthe string, is different from a number of the second plurality ofphotovoltaic panels.
 25. The system of claim 18, wherein each of theplurality of power converters comprises one of: buck convertercircuitry; boost converter circuitry; buck-boost converter circuitry; orbuck plus boost converter circuitry.
 26. The system of claim 18, whereineach of the plurality of power converters is configured to selectivelyoperate in at least one of a buck conversion mode, a boost conversionmode, or a buck-boost conversion mode.
 27. The system of claim 23,wherein each of the at least two of the plurality of photovoltaic panelsis integrated with a roof, a window, or a wall.
 28. The system of claim23, wherein each of the at least two of the plurality of photovoltaicpanels is a roof tile.
 29. The system of claim 18, wherein the at leasttwo of the plurality of photovoltaic panels are associated withdifferent facets of a building.
 30. A system comprising: a stringcomprising: a plurality of serially connected photovoltaic panels; and apower converter coupled across the plurality of serially connectedphotovoltaic panels, wherein: the power converter is configured to drawpower from the plurality of serially connected photovoltaic panelsaccording to a maximum power point tracking (MPPT) algorithm, and atleast two photovoltaic panels of the plurality of serially connectedphotovoltaic panels are building integrated photovoltaic panels.
 31. Thesystem of claim 30, further comprising an inverter connected to thestring, wherein the inverter is configured to regulate a common voltageacross inputs of the string.
 32. The system of claim 30, wherein thepower converter is configured to limit, based on a condition, an outputvoltage of the power converter to a safe level.
 33. The system of claim32, wherein the condition is one of a predetermined minimum load, or asignal sent from an inverter.
 34. The system of claim 31, furthercomprising a battery connected to the inverter, wherein the inverter isconfigured to charge the battery with power provided by the string. 35.The system of claim 30, further comprising a second string, the secondstring comprising: a second plurality of serially connected photovoltaicpanels; and a second power converter coupled across the second pluralityof serially connected photovoltaic panels, wherein: the second powerconverter is configured to draw power from the second plurality ofserially connected photovoltaic panels according to a maximum powerpoint tracking (MPPT) algorithm, and at least two photovoltaic panels ofthe second plurality of serially connected photovoltaic panels arebuilding integrated photovoltaic panels.
 36. The system of claim 35,where a number of the plurality of serially connected photovoltaicpanels is different from a number of the second plurality of seriallyconnected photovoltaic panels.
 37. The system of claim 30, wherein eachof the at least two photovoltaic panels is integrated with a roof, awindow, or a wall.
 38. The system of claim 30, wherein each of the atleast two photovoltaic panels is a roof tile.
 39. A photovoltaic stringcomprising: a plurality of photovoltaic panels; and a plurality of powerconverters connected in series, wherein: each one of the plurality ofpower converters is connected to a corresponding one of the plurality ofphotovoltaic panels, each one of the plurality of power converters isconfigured to draw power from the corresponding one of the plurality ofphotovoltaic panels according to a corresponding maximum power pointtracking (MPPT) algorithm, and at least two of the plurality ofphotovoltaic panels are building integrated photovoltaic panels.